In the field of genetic engineering, phages, animal or plant viruses, plasmids, etc. are widely used as expression vectors for transforming microorganisms. As transformant microorganisms that are transformed and made to express a target protein of a gene product, E. Coli, yeast, etc. are widely used. These transformed microorganisms are aimed at expressing a target protein, and utilization of the microorganisms themselves is not contemplated.
In recent years, with regard to utilization of a transformed microorganism itself, a method for treating a malignant tumor has been attracting an attention in which a transformed anaerobic bacterium is used as a gene transporter; for example, a method of transporting a gene to a tumor site using transformed Clostridium (see e.g. Patent Publications 1 to 3) has been proposed and, furthermore, application of transformed Bifidobacterium longum to the treatment of solid tumors has been suggested (see e.g. Nonpatent Publications 1 and 2).
Furthermore, with regard to a transformed Bifidobacterium useful as a gene transporter for treatment of a solid tumor, it has been reported that Bifidobacterium longum transformed so as to express cytosine deaminase (hereinafter, called CD) can be expected to have an application in an enzyme-prodrug therapy (see e.g. Patent Publication 4 and Nonpatent Publications 3 and 4). CD is an enzyme that converts 5-fluorocytosine (hereinafter, called 5-FC), which is a prodrug (precursor) of 5-fluorouracil (hereinafter, called 5-FU) that has an antitumor activity, into 5-FU.
Construction of such a transformed bacterium requires an expression vector. However, since an E. coli-derived plasmid vector conventionally used for transforming E. coli in the field of genetic engineering is naturally unable to be replicated in bacteria other than E. coli, it is necessary in the construction of the transformed bacterium to modify a plasmid vector so that it is capable of being replicated in the transformed bacterium.
In the above publications, expression vectors used in the construction of such transformed bacteria for treatment of a malignant tumor have also been reported, and Patent Publications 1 to 3 report the shuttle plasmids pNTR500F, pCD540FT, etc., which are replicated in both E. coli and Clostridium. 
Furthermore, Patent Publication 4 reports the shuttle plasmid pBLES 100-S-eCD, and the shuttle plasmid pBLES 100 used for construction of the shuttle plasmid pBLES100-S-eCD, which are replicated in both E. coli and Bifidobacterium. 
In addition, the shuttle plasmids pAV001-HU-eCD, which can transform Bifidobacterium longum at a high efficiency of more than 100 times that of the shuttle plasmid pBLES100-S-eCD, have also been reported (see e.g. Patent Publication 5).
Furthermore, the shuttle plasmid pAV001-HU-eCD-M968, which is a plasmid single-nucleotide variant of the shuttle plasmid pAV001-HU-eCD in which the DNA of the target gene inserted into the shuttle plasmid pAV001 has been partially varied, has been reported (see e.g. Patent Publication 6).
Furthermore, for example, the shuttle plasmid pDG7, which is replicated in both E. coli and Bifidobacterium, the shuttle plasmids pEBM3 and pECM2, which are replicated in both E. coli and Clostridium, the shuttle plasmid pLP825, which is replicated in both E. coli and Lactobacillus, etc. have been reported (see e.g. Nonpatent Publication 5).
As hereinbefore described, various plasmid vectors used for constructing a transformant other than E. coli have been reported, they are all shuttle vectors that are replicated in both E. coli and a transformant bacterium other than E. coli, and there is no known plasmid vector that is capable of being replicated only in a non-E. coli transformant bacterium.
[Patent Publication 1] U.S. Pat. No. 6,416,754
[Patent Publication 2] U.S. Pat. No. 6,652,849
[Patent Publication 3] US Pat. Laid-open No. 2003/0103952
[Patent Publication 4] JP, A, 2002-97144
[Patent Publication 5] WO 2006-57289
[Patent Publication 6] WO 2007-136107
[Nonpatent Publication 1] Yazawa et al., Cancer Gene Ther., 7, 269-274 (2000)
[Nonpatent Publication 2] Yazawa et al., Breast Cancer Res. Treat., 66, 165-170 (2001)
[Nonpatent Publication 3] Nakamura et al., Biosci. Biotechnol. Biochem., 66, 2362-2366 (2002)
[Nonpatent Publication 4] Fujimori et al., Curr. Opin. Drug Discov. Devel., 5, 200-203 (2002)
[Nonpatent Publication 5] Alessandra Argnani et al., Microbiology.; 142: 109-114 (1996)